1. Field of the Invention
The present invention generally relates to a technique for measuring an intensity of an electron beam, and more particularly relates to an apparatus for measuring an intensity of an electron beam and to an electron microscope including the apparatus for measuring an intensity of an electron beam.
2. Related Art Statement
There has been proposed a transmission type electron microscope comprising an electron gun producing an electron beam, a condenser lens system for converging the electron beam and projecting the thus converged electron beam onto a specimen, an objective lens system for forming an electron beam image of the specimen, an enlarging lens system for forming an enlarged electron beam image of the specimen and projecting the enlarged electron beam image, a fluorescent screen converting the enlarged electron beam image of the specimen into an optical image of the specimen, and a shutter arranged between the enlarging lens system and a photographic film.
In such a transmission type electron microscope, contract and brightness vary over a very wide range, and therefore it is necessary to provide a certain index for projecting an optimum electron beam image of the specimen onto an image record medium, for example, a photographic film. In a remote observing system in which an optical image of a specimen is observed with a television camera, it is particularly difficult to judge an optimum exposure accurately based on a brightness of the fluorescent screen through experiences, because the brightness of the optical image changes in accordance with a sensitivity of the television camera. It is important for the transmission type electron microscope to provide a precise control for the exposure by means of which an operator can have a confidence that excellent photographs can be obtained. It has been proposed to arrange temporally a detector in front of the optical image monitoring fluorescent screen only when an intensity of the electron beam is to be measured. It has been further proposed to measure an intensity of an electron beam flowing into the fluorescent screen by means of a plate electrode built-in the screen.
In the former method, an optical image of a specimen could not be monitored by the television camera during the electron beam intensity measurement, because the detector is arranged in front of the fluorescent screen. That is to say, both the observation of the optical image and the measurement of the electron beam intensity could not be performed simultaneously. The latter method has been generally used in conventional electron microscopes having low electron beam energy, but could not be applied to ultra-high voltage electron microscopes using an electron beam having high energy owing to a reason that a detected current is no more proportional to an intensity of the electron beam. This is due to a fact that a difference between electrons trapped by the fluorescent screen and secondary electrons emitted from the florescent screen is detected. Therefore, in an extreme case, a polarity of the detected current might be reversed, and the detected signal could not be used any more.
In order to perform the measurement of the intensity of the electron beam effectively for a diffraction image and an electron beam image having a high contrast, it is preferable to apply the center preponderant measurement system, almost equal to the spot measurement.